In recent years, much attention has been focused on novel power generating techniques which are superior in energy efficiency or environmental friendliness. In particular, solid polymer fuel cells using solid polymer electrolyte membranes are characterized as exhibiting high energy density and being started and stopped more easily than fuel cells of other systems due to their lower operating temperature. Therefore, they are on development as generators for electric motorcars, dispersed power generation and the like. In addition, development of direct methanol fuel cells which use solid polymer electrolyte membranes and into which methanol is supplied directly as fuel are underway for applications such as electric sources of portable devices.
Membranes comprising proton-conducting ion exchange resin films are usually used for solid polymer electrolyte membranes. Solid polymer electrolyte membranes are required to have characteristics such as fuel permeation inhibitability and mechanical strength preventing permeation such as hydrogen and methanol of fuel is necessary for a solid polymer electrolyte membrane as well as proton conductivity. As such a solid polymer electrolyte membrane, for example, perfluorocarbon sulfonic acid polymer membranes in which sulfonic acid groups are introduced, typified by Nafion (registered trademark) made by Du Pont, U.S.A., are known.
However, because membranes comprising perfluorocarbon sulfonic acid polymer soften up at temperatures of 100° C. or higher, the operating temperature of fuel cells using such membranes is limited to 80° C. or lower. Heat-resistant solid polymer electrolyte membranes have been studied because increase of operation temperature leads to various advantages, for example, energy efficiency, miniaturization of apparatuses and improvement of catalytic activity.
In recent years, as an alternative solid polymer electrolyte membrane to membranes containing perfluorocarbon sulfonic acid polymer, active investigations have been made to so-called hydrocarbon-based polymer solid electrolytes, which contain polymers resulting from introduction of ionizable groups such as a sulfonate group into polyetheretherketone polymers, polyethersulfone polymers, polysulfone polymers, etc.
One example thereof is a membrane containing polysulfone having a sulfonate group (see, for example, F. Lufrano and three other authors, “Sulfonated Polysulfone as Promising Membranes for Polymer Electrolyte Fuel Cells” Journal of Applied Polymer Science (U.S.A.), John Wiley & Sons, Inc., 2000, Vol. 77, pp. 1250-1256). Polysulfone is suitable as a raw material of solid polymer electrolyte membranes because it is superior in processability; for example, it has a high heat resistance and it is soluble in organic solvents. A sulfonic acid group is usually introduced into polysulfone by use of a sulfonating agent such as concentrated sulfuric acid and sulfuric anhydride. It is however difficult to control sulfonation reactions by this method. In some cases, therefore, it is impossible to adjust the degree of sulfonation to a desired degree or problems such as gelation are caused by nonuniform sulfonation or side reactions.
Hydrocarbon-based solid polymer electrolytes including the above-mentioned suflonic acid group-introduced polysulfone have problems with respect to water resistance under high humidity because they are more prone to hydration or swelling in comparison to membranes containing perfluorocarbon sulfonic acid polymers.
As one measure for inhibiting such swelling, a technique using mixing with a basic polymer has been investigated. This technique tries to inhibit the swelling by crosslinking sulfonic acid groups in a solid polymer electrolyte membrane with a basic polymer. Examples thereof include a technique using a mixture of a polyethersulfone-based polymer having a sulfonic acid group or a polyetheretherketone-based polymer (acid polymer) and a polybenzimidazole-based polymer (basic polymer) (see, for example, WO 99/54389 pamphlet).
In addition, a technique to inhibit swelling by crosslinking between sulfonic acid groups, which are ionizable groups, with a covalent bond is also investigated (see, for example, Japanese Laid-Open Patent Publication No. 6-93114 (U.S. Pat. No. 5,438,082, EP0 574 791 B1), WO 99/61141 pamphlet, and WO 99/38897 pamphlet).
All the above techniques, however, can inhibit swelling, but they are problematic in that ionizable groups lose their ionicity through the crosslinking reaction and, as a result, ion conductivity falls.
As solid polymer electrolyte membranes having a crosslinked structure, membranes containing a sulfonated product from a styrene/divinyl benzene copolymer are well known for their use in early solid polymer-type fuel cells. Such solid polymer electrolyte membranes, however, did not exhibit satisfactory characteristics as fuel cells because their polymer skeleton itself was poor in durability.
Moreover, another technique is regarding an ion exchange product obtained by subjecting chloromethyl groups in a polymer to a crosslinking polymerization using a Lewis acid as a catalyst (see, for example, Japanese Laid-Open Patent Publication Nos. 2-248434 and 2-245035). The crosslinking reaction of this technique, however, requires a catalyst. Therefore, when obtaining a molding of ion exchange product by mixing a polymer and a catalyst, the remaining of the catalyst becomes a problem. In addition, when obtaining a molding of an ion exchange product by treating a molding of a polymer with a catalyst, the difficulty in occurrence of a crosslinking reaction inside the molding of the polymer becomes a problem.
Thus, a technique including synthesizing a sulfonated polymer by polymerizing monomers having a sulfonic acid group instead of sulfonating an existing polymers and using it as a solid polymer electrolyte is under investigation (see, for example, Japanese Laid-Open Patent Publication No. 5-1149 and U.S. Patent Unexamined Application Publication No. 2002/0091225 specification). These sulfonated polymers are advantageous because their degrees of sulfonation can be adjusted easily and it is easy to obtain their uniform solutions. When a solid polymer electrolyte is used as an ion exchange membrane, in particular, when it is used as a proton exchange membrane of a fuel cell, the higher the ion conductivity of the membrane, the better the performance. Therefore, the ion conductivity increases as the sulfonic acid group concentration in the membrane is increased. Among the aforementioned sulfonated polymers, however, those having high degrees of sulfonation swell greatly. Therefore, when they are used as proton exchange membranes of fuel cells, problems tend to occur such as crossover and crossleak of gas, delamination and breakage of electrodes, etc.
Therefore, a technique to improve the mechanical strength of solid polymer electrolyte membranes to inhibit the dimensional change thereof by combining various reinforcing materials with solid polymer electrolyte membranes is under investigation. As one example thereof, reinforcement by blending a sulfonated polymer resulting from polymerization of sulfonated monomers and a non-sulfonated polymer possessing a similar structure has been proposed (see, for example, Japanese Laid-Open Patent Publication No. 5-4031). It, however, has drawbacks in that the sulfonated polymer and the non-sulfonated polymer are less compatible with each other due to a great difference between their polarities and, therefore, it is impossible to obtain uniform membranes.
In addition, reinforcement of a sulfonated polymer with a porous support membrane has also been proposed (see, for example, WO 00/22684 pamphlet). However, as the sulfonated polymer, only existing polymers are listed and this publication discloses no example of using a sulfonated polymer, which is a better polymer electrolyte, obtained by polymerization of sulfonated monomers. In addition, the support disclosed in this publication has a drawback in that if it is fabricated into a composite membrane, the ion conductivity will fall due to the low porosity of the support membrane.
Based on the circumstances, a major object of the present invention is to provide a composite ion exchange membrane having a high swelling resistance and being superior in mechanical strength and ion conductivity.